Type classes

Philip Wadler


A second look at overloading

Martin Odersky, Philip Wadler, Martin Wehr. 7'th International Conference on Functional Programming and Computer Architecture, ACM Press, San Diego, California, June 1995.

We study a minimal extension of the Hindley/Milner system that supports overloading and polymorphic records. We show that the type system is sound with respect to a standard untyped compositional semantics. We also show that every typable term in this system has a principal type and give an algorithm to reconstruct that type.

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Type classes in Haskell

Cordelia Hall, Kevin Hammond, Simon Peyton Jones, and Philip Wadler. European Symposium On Programming, LNCS 788, Springer Verlag, pp. 241-256, April 1994.

This paper defines a set of type inference rules for resolving overloading introduced by type classes. Programs including type classes are transformed into ones which may be typed by the Hindley-Milner inference rules. In contrast to an other work on type classes, the rules presented here relate directly to user programs. An innovative aspect of this work is the use of second-order lambda calculus to record type information in the program.

Available in: dvi, ps, dvi.gz, ps.gz.


A static semantics for Haskell

Simon Peyton Jones and Philip Wadler. Draft paper, Glasgow, 1991.

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How to make ad-hoc polymorphism less ad hoc

Philip Wadler and Stephen Blott. 16'th Symposium on Principles of Programming Languages, ACM Press, Austin, Texas, January 1989.

This paper presents type classes, a new approach to ad-hoc polymorphism. Type classes permit overloading of arithmetic operators such as multiplication, and generalise the ``eqtype variables'' of Standard ML. Type classes extend the Hindley-Milner polymorphic type system, and provide a new approach to issues that arise in object-oriented programming, bounded type quantification, and abstract data types. This paper provides an informal introduction to type classes, and defines them formally by means of type inference rules.

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Philip Wadler,